1. Field of the Invention
The present invention relates to a cage for the intercorporal fusion of lumbar vertebrae.
The technical field to which the present invention relates is spinal surgery. To treat people with degenerative changes to the lumbar vertebral column, in which a mono- or multi-segment fusion is indicated, but also in cases which are aimed at a decompression of the neural structures, two operating techniques in particular have proven effective, which enable intervention without additional trans-abdominal access. In the so-called PLIF operating technique (Posterior Lumbar Intervertebral Fusion) two so-called cages are preferably implanted for each disc space by way of posterior access. The implanted cages serve as placeholders in the intervertebral space and restore the spacing of the adjacent vertebral bodies as desired by the surgeon. In addition, the vertebrae are therefore stabilised in a fixed position by a rod-screw system applied posteriorly. The second operating technique, known as TLIF (Transforaminal Lumbar Intervertebral Fusion) is an operating method which has been further developed from the PLIF operating technique. In this procedure, a single cage is implanted via dorsolateral access. Posterior stabilisation is also applied with this technique. The aim of both operating techniques is to achieve a fusion, i.e. an osseous connection, of the affected segments. This can be improved by the addition of a synthetic or a biological bone-substitute material. The cages are normally made from metal or plastic materials.
2. Description of Related Art
For both operating methods, the prior art discloses cages which are approximately trapezoidal or wedge-shaped in longitudinal section with a rectangular cross-section, which cages correct the curvature of the vertebral column as result of their shape. So-called expandable cages of the said type are also already known, and these offer the advantage of enabling relatively easy insertion owing to the low entry height.
EP 1 290 985 B1 discloses an expandable cage of this type which has an expansion element inside it for expanding the cage, wherein a spacer disc, which is movable on slide rails with angled slide surfaces, can be inserted so that, depending on the depth of insertion of the spacer element, the cage expands to varying degrees as far as an end position formed by a projection or a step.
Although this known cage offers the above-mentioned advantage owing to its expandability, the necessary procedure of inserting the spacer disc as far as the end position for expanding the cage is, however, complex and requires a high level of ability and skill on the part of the surgeon. When a cage is only half-expanded, the spacer element is located in the centre of the cage, the front ends protrude and a reliable load transfer is not ensured. Moreover, the spacer disc has to be positively guided to prevent tilting. It remains equally difficult to clearly control the degree of expansion which is achieved and to monitor whether the end position has been reliably reached.
DE 101 13 689 C1 discloses a generic expandable cage which prevents the above-mentioned disadvantages in that the expansion of the cage is effected by a rotatable expansion element mounted at its front end. To this end, this cage has two substantially mutually parallel-extending arms which form supporting surfaces for adjacent vertebral bodies on their outsides and are connected at one end by way of a bridge. At the respective other ends of the arms, there is an intermediate elliptical expansion element, which engages in the ends of the arms and is rotatable about an axis of rotation extending parallel to the arms. This enables the arms to be expanded to a greater or lesser extent depending on the angular position of the expansion element. In the unexpanded state of the cage, the outer surfaces of the expansion element with the smaller radius of curvature rest in oval cutouts in the arms. In the expanded state, the outer surfaces of the expansion element with the larger radius of curvature each lie in depressions in the arms, which are delimited on both sides by small steps. The expansion element can be inserted from the outside into groove-shaped depressions in both arms, so that it can be secured in this manner against displacement in the axial direction.
Although more simple and rapid adjustability of the cage into the expanded position can be achieved in this way, the limited contact surface between the expansion element and the arms in the expanded state means that the load-bearing capacity is low, especially since, when subjected to a load, it is not possible to reliably prevent the expansion element from rotating about the axis of rotation and the arms from tilting about axes lying parallel to the axis of rotation.
U.S. Pat. No. 6,723,128 B2 discloses a monolithically constructed expandable cage having a rotatable expansion element which is incorporated flush between two arms in an insertion opening and whereof the respective contact surfaces supporting the arms are widened and therefore enable a greater load transfer. However, when subjected to a load, it is still possible for the arms to tilt about axes lying parallel to the longitudinal axis of the cage since the width of the contact surfaces, which is transverse to the longitudinal axis of the cage, is relatively small by comparison with the width of the cage. It is also not possible to rule out the expansion element itself tilting about a transverse axis which is perpendicular to the longitudinal axis of the cage. The expansion path is restricted.
EP 1 233 732 B1 discloses an expandable cage with two expandable arms, which are optionally also connected to one another in hinged manner and which have a rotatable expansion element located between them. In a preferred form, the plate-shaped expansion element has a modified rectangular or rhomboidal form with diagonally opposed rounded corners in order to facilitate the rotation of the expansion element when transferring from the unexpanded to the expanded state of the cage and to prevent a substantial over-distraction of the adjacent vertebral bodies during this procedure. The expansion element is held and guided in guides of the two expandable arms. Although it is also possible here for the expansion element to tilt about a transverse axis which is perpendicular to the longitudinal axis of the cage, a tilting of the arms about an axis lying parallel to the longitudinal axis of the cage is prevented by complex stabilising partial wall structures of the arms themselves. These not only increase the rigidity of the arms but are at the same time also restrictive in terms of allowing the largest possible lateral recesses in the cage, which are advantageous for improved radiological penetration.